Operations of lean burn engines, for example diesel engines, provide the user with excellent fuel economy due to their operation at high air/fuel ratios under fuel lean conditions. However, diesel engines also emit exhaust gas emissions containing particulate matter (PM), unburned hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), wherein NOx describes various chemical species of nitrogen oxides, including nitrogen monoxide and nitrogen dioxide, among others.
Oxidation catalysts comprising precious metals, such as gold, platinum, palladium, rhodium, iridium, ruthenium and osmium, dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both hydrocarbon (HC) and carbon monoxide (CO) gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts may be contained in diesel oxidation catalysts (DOC), which are placed in the exhaust flow path from a diesel powered engine to treat the exhaust gas stream. Typically, the diesel oxidation catalysts are prepared on ceramic or metallic carrier substrates upon which one or more catalyst coating compositions are deposited. In addition to the conversion of gaseous HC, CO and the soluble organic fraction of particulate matter, oxidation catalysts containing precious metals dispersed on a refractory oxide support may promote the oxidation of nitric oxide to nitrogen dioxide.
As is well-known in the art, catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation, because the engine exhaust is not at a temperature sufficiently high for efficient catalytic conversion of noxious components in the exhaust. To this end, it is known in the art to include an adsorbent material, which may be a zeolite, as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocarbons, and retain them during the initial cold-start period. As the exhaust gas temperature increases, the adsorbed hydrocarbons are driven from the adsorbent and subjected to catalytic treatment at the higher temperature.
As mentioned, oxidation catalysts comprising a precious metal dispersed on a refractory metal oxide support are known for use in treating exhaust gas emissions from diesel engines. Platinum (Pt) remains the primary platinum group metal for oxidizing CO and HC in a DOC, after high temperature aging under lean conditions. One of the major advantages of using palladium (Pd) based catalysts is the lower cost of palladium compared to platinum. However, while addition of palladium to platinum based DOCs does inhibit sintering of platinum and improve CO and HC oxidation performance after high temperature aging, having too much palladium may decrease the activity of platinum to convert paraffins and/or oxidize nitric oxide, especially when used with HC storage materials, and may also make the catalyst more susceptible to sulfur poisoning. These characteristics have typically prevented the replacement of Pt by Pd as an oxidation catalyst in lean burn operations especially for light duty diesel applications, where engine temperatures remain below 250° C. for most driving conditions.
In addition, current diesel engines utilizing new advanced combustion technologies such as Homogeneous Charge Compression Ignition (HCCI) are able to reduce engine output of NOx and particulate matter (PM) emissions by reducing the combustion flame temperature within the engine cylinder and by increasing the uniformity and mixing of the fuel charge prior to ignition. However, in the process of changing the combustion process to lower NOx and PM emissions, the overall quantity of CO and HC emissions can increase, the nature of the HCs formed can change, and the exhaust temperature may be lowered. In some instances, the CO and HC emissions from advanced combustion diesel engines is 50% to about 100% higher than the HC and CO emissions from traditional diesel engines. Furthermore, as vehicle manufacturers seek to meet long term worldwide fuel economy standards, the engine exhaust temperature is expected to decline significantly, thereby challenging the DOC to function at lower and lower temperature to oxidize CO, HC and NOx. DOC catalysts with lower light-off for CO and HC will be required.
These observations, in conjunction with emissions regulations becoming more stringent, has driven the need for developing emission gas treatment systems with improved CO and HC oxidation capacity to manage CO and HC emissions at low engine exhaust temperatures.